1. Field of the Invention
The present invention concerns navigation and collision-avoidance of aircraft, and particularly a method of using an on-board locating system to passively compute, monitor, and display the positions of own and other aircraft.
2. Description of Prior Art
The need for airborne devices to help pilots avoid midair collisions has been recognized for several years. The July-August 1984 edition of "FAA General Aviation News" reported that "over the past quarter century, midairs have averaged over 25 per year, with an overall total of 1550 fatalities. . . In 1982, the latest year for which the National Transportation Safety Board has complete figures, there were 36 midairs, resulting in 59 fatalities." Most midair collisions involve military or general aviation aircraft. Those that involve commercial flights, such as the Sept. 25, 1978 collision over San Diego Calif., which resulted in 144 deaths, typically attract the greatest coverage by the news media.
An early airborne collision-avoidance system, proposed in the 1970s, was not accepted by the Federal Aviation Administration (FAA), largely because it would have protected appropriately equipped aircraft only from other aircraft carrying a similar collision-avoidance device. Instead, the FAA concentrated its efforts on systems that rely on detecting aircraft equipped with secondary radar transponders. More than half of the U.S. civil aviation fleet is transponder-equipped; and most of the flying done in the United States is done by transponder-equipped craft. Therefore, a collision-avoidance device that detects proximate transponder-equipped aircraft can protect its own aircraft from a large number of others. The invention described herein also follows this approach, in that it relies on the existing secondary radar system to provide electromagnetic signals which allow any suitably equipped aircraft to passively detect transponder-equipped aircraft that are close to it.
Secondary radars, also called beacon radars, have been used for many years to gather surveillance data for air traffic controllers. Within the FAA they are formally known as Air Traffic Control Beacon Interrogators (ATCBI). There are several types, the ATCBI-3, ATCBI-4, and ATCBI-5 currently being used. Each differs from the others in details depending mainly on the manufacturer and the technology available at the time of its design. Internationally, beacon radars are formally known as "secondary surveillance radars." The United States standards for secondary radars and the airborne transponders that work with them are given by the U.S. National Standard for Mark X (SIF) Air Traffic Control Radar-Beacon System Characteristics, which is attachment 1 to Federal Aviation Administration Order 1010.51A. The international standards for secondary radars and transponders are in Volume 1 of Annex 10 to the International Civil Aviation Organization (ICAO) Convention. The U.S. standards agree with the international standards. The well-known reference Radar Handbook, edited by Merrill I. Skolnik (McGraw-Hill Book Company Inc., 1970; Library of Congress catalogue card number 69-13615) includes a description of the secondary radar system as its chapter 38, "Beacons."
Secondary radars should not be confused with primary radars. The two are frequenly co-located; their outputs are often combined before being transmitted to a computerized air traffic control facility; they occasionally even use the same antenna. However, they operate by different principles, and are designed and built differently. A primary radar periodically transmits a very powerful pulse of rf energy, and then detects targets by receiving their reflections of the transmitted pulse. A secondary radar periodically transmits a pair of rf pulses at relatively low power. When an aircraft transponder receives the pulse pair, it replies with a train of rf pulses that are then received by the secondary radar. The secondary radar system is thus a cooperative system. Primary radars can be used to detect non-cooperating targets, such as enemy aircraft, civil aircraft without transponders, and severe storms. Though useful for defense, weather detection, and air traffic control, primary radars are not relevant to the present invention. Therefore, any further reference herein to a radar signifies a secondary radar. The following material summarizes the operation of the secondary radar system as presently used, illustrating the relationships between groundbased radars and airborne transponders.
FIG. 1 depicts an aircraft equipped with a secondary radar transponder, 10. The aircraft is flying in the vicinity of a radar, 12. Directional antenna 14 of radar 12 rotates continuously about a vertical axis at a uniform rate. Rotation rates (scan rates) such as 5 or 6 revolutions per minute (rpm) are typical for long-range radars, having ranges of approximately 200 nautical miles. Scan rates such as 12 or 15 rpm are typical for terminal area radars, having ranges of approximately 60 nautical miles. As antenna 14 rotates, it emits a pulsed beam 16 of electromagnetic radiation at a frequency of 1030 MHz. The beam consists of interrogating pulse pairs 18 (interrogations), transmitted at a predetermined average pulse repetition frequency (prf), typically between 300 and 450 interrogations per second. (Some of the newest models of secondary radar, those with "monopulse" antennas and processors, will operate at lower prf, typically 150 interrogations per second.) The horizontal beam width is typically between 2.5 degrees and 3.5 degrees, measured at the half-power points. An interrogation 18 consists of pulses named P1 and P3, also shown in FIG. 2. Each pulse lasts 0.8 microseconds; and the leading edge of pulse P3 follows that of pulse P1 by either 8 or 21 microseconds. Interrogations with 8-microsecond spacing are called mode A interrogations; those with 21-microsecond spacing are called mode C interrogations.
An aircraft transponder determines the mode of an interrogation by measuring the time between the 1030 MHz pulses it receives. When the transponder receives a mode A interrogation, it replies with a code representing its aircraft's identity. When it receives a mode C interrogation, it replies with a code representing its aircraft's altitude. A beacon radar's interrogations follow a repeated mode interlace pattern, such as alternating mode A and mode C interrogations, or two consecutive mode A interrogations followed by a mode C interrogation.
In FIG. 1, aircraft transponder 10 will reply to beacon-radar interrogation 18, 3 microseconds after receiving the leading edge of the P3 pulse. That 3-microsecond time is part of the transponder's "turnaround" time, herein called t.sub.t. The reply consists of electromagnetic pulses, omnidirectionally transmitted at a frequency of 1090 MHz from the aircraft's transponder. FIG. 3 shows a typical train of such pulses, and in particular, shows the framing pulses, F1 and F2, which are always present in the reply, their leading edges spaced 20.3 microseconds apart.
The presence or absence of a pulse in each of twelve defined positions between the framing pulses (the "X" position shown in FIG. 3 is not used by civil aviation transponders) determines one bit of information in the aircraft's coded reply. The reply thus consists of twelve bits, which are normally viewed as being grouped into 4 octal digits called A, B, C, and D. The transmitted code is the 4-digit sequence ABCD. In FIG. 3, the numerical suffix of each pulse position indicates the contribution to the value of its octal digit made by the presence of a pulse in that position. For example, the presence of pulses in positions B1 and B4, and the absence of a pulse in position B2, indicates that the value of octal digit B is 1.times.1+O.times.2+1.times.4=5. All reply pulses are 0.45 microseconds long. Adjacent position are separated by 1 microsecond. Thus, in FIG. 3, the leading edge of the C4 pulse follows that of the A2 pulse by 1.45 microseconds; likewise the leading edges of the B4 and D2 pulses are 1.45 microseconds apart.
Antenna 14 in FIG. 1 rotates in a clockwise direction as shown by arrow 20, and irradiates objects with beam 16. The time during which beam 16 irradiates an object is known as the beam dwell, and depends primarily on the rotation rate and the beam width. A 3-degree radar beam rotating at the rate of 4 seconds per revolution will irradiate a transponder for 1/30 second during each revolution. If the prf is, for example, 300 interrogations per second, then the transponder will be interrogated approximately 10 times during the beam dwell. In practice, the beam width that a transponder "experiences" varies somewhat, depending on such factors as the radar's output power and the transponder's sensitivity.
FIG. 1 shows that each time radar 12 emits an interrogation, it also omnidirectionally transmits a pair of pulses 22 for "improved sidelobe suppression" (ISLS). (Formerly a separate antenna was used to transmit the ISLS pair; however, the FAA's newest beacon radar antenna, the "five-foot open array," formally known as type FA-9764, transmits both the directional interrogation 18 and the omnidirectional ISLS pair 22.) The pulses in the ISLS pair are called P1 and P2; and the ISLS P1 pulse is transmitted simultaneously with the interrogation P1 pulse. As shown in FIG. 4, each pulse lasts 0.8 microseconds; and the leading edge of the P2 pulse follows that of the P1 pulse by 2 microseconds. P1 and P2 have the same amplitude, which must at least equal the amplitudes of pulses transmitted by the strongest sidelobe of directional beam 16.
The ISLS pulse pair is transmitted to eliminate unwanted transponder replies, which can be triggered by a transponder's receipt of e.threr a sidelobe of interrogating beam 16, or a reflection of beam 16. Both the U.S. and international standards for secondary radar require transponders to compare the amplitudes of received pulses, and loc up for 35 microseconds unless P1 is significantly stronger than P2.
A transponder directly interrogated by beam 16 will receive P1 at much greater amplitude than P2, because of the far greater gain of directional beam 16. The transponder will therefore reply normally.
A transponder receiving a sidelobe of beam 16 will find the amplitude of P2 to be at least that of P1, and will lock up. A transponder receiving a reflection of beam 16 will have already received the simultaneous ISLS transmission, since the reflection must have traveled farther to reach the transponder. Having found
P1 and P2 to be of equal amplitude, the transponder will have locked up, and be incapable of replying to the reflection.
There are many places where an aircraft may be in overlapping coverage areas of several secondary radars. At times, an aircraft may be in the surveillance beams, 16, of two or more of those radars. When a transponder determines that it has received an interrogation, it ignores all other interrogations that may reach it until it has completed its reply. Thus if the interrogations from one radar arrive at a transponder a few microseconds before those of a second radar, the first radar prevents the transponder from replying to the second. In fact, the second radar is likely to receive the transponder's replies to the first radar's interrogations, and, basing its computation on the time it transmitted its own interrogation, miscalculate the distance from itself to the aircraft.
Two techniques are often used to prevent such interference problems. In the United States, the FAA varies the pulse repetition frequency (prf) of neighboring radars, so that their interrogations are not synchronous. Therefore, even if a particular interrogation from one radar prevents a reply to an interrogation from a second radar, their next few interrogations will reach the aircraft sufficiently separated in time to avoid locking out each other.
The other technique, also used by the FAA, extends this approach by changing the interpulse period--i.e. by "staggering" the interrogations. A radar with a prf of, for example, 400 interrogations per second, has an average inter-interrogation period of 1/400 second, or 2500 microseconds. Over a long period of time, the radar maintains that average prf; however, over a time in which, for example, 6 or 7 interrogations are transmitted, the time between successive P1 pulses can vary from, for example, 1800 microseconds to 4000 microseconds. The variation follows a fixed pattern, the "stagger pattern," which is continually repeated. Neighboring radars use different patterns. Therefore, even if two radars' beams illuminate a transponder during the same period, their interrogations will not be synchronous, and both will receive replies from the transponder.
In order to monitor the performance of a beacon radar, the FAA frequently installs a transponder on a conveniently located building or tower, where it is "visible" to the radar. The transponder's position is fixed and known, and technicians can monitor the radar's reports of the transponder's position. If the reports disagree with the known position, technicians can then check the radar to determine if its operation is faulty. The present invention also uses transponders fixed in positions visible to beacon radars; however it uses them for an entirely different purpose.
Work on airborne collision-avoidance devices has been underway for several years, most of it sponsored or performed by the FAA and its contractors. In June 1981, the Administrator of the FAA, Mr. J. Lynn Helms, publicly announced that the FAA would establish standards for two levels of airborne collision-avoidance devices, which became known as Traffic Alert and Collision Avoidance Systems (TCAS). The TCAS-I would be designed to operate in most general aviation aircraft, while the more sophisticated and more expensive TCAS-II would be designed for airliners and high-performance general aviation craft. In his announcement that the FAA would promulgate TCAS minimum standards, Mr. Helms encouraged private industry to not only build equipment meeting those standards, but also to incorporate improvements compatible with them. The present invention consists of a method and an apparatus that are entirely compatible with the standards established to date by the FAA and major avionics manufacturers. The FAA and the manufacturers have worked together through the Radio Technical Commission for Aeronautics (RTCA). The TCAS-I Functional Guidelines is document RTCA/DO-184; and the Minimum Operational Performance Standards for Traffic Alert and Collision Avoidance System Airborne Equipment is document RTCA/DO-185 (2 volumes). The U.S. National Aviation Standard for the Traffic Alert and Collision Avoidance System II is FAA Order 6367.1.
In its simplest form, the TCAS-I operates passively in its detection of another aircraft. It receives the other aircraft's transponder replies to secondary radar interrogations, and estimates the other aircraft's distance from itself by measuring the strength of the replies. A device using this principle was available during the 1970s, but was not widely accepted by the aviation community. Distance measurement by this method is not very accurate. Another limitation of the simplest form of TCAS-I is that it will not offer the pilot any directional guidance to help him locate threatening aircraft. The method described herein will locate aircraft with an accuracy comparable to that of the secondary radar system, thus overcoming major shortcomings of TCAS-I.
The TCAS-II developed by the FAA will utilize a directional antenna to interrogate other aircraft, operating essentially as an airborne secondary radar. While this method of operation will enable it to accurately measure the distance from its own aircraft to a proximate transponder-equipped aircraft, the directional information obtained by the minimum TCAS-II will only be accurate to between five and ten degrees. That accuracy is not considered sufficient for computation of horizontal collision-avoidance maneuvers. The enhanced TCAS-II, using a more sophisticated and more expensive antenna, should achieve two-degree accuracy. The invention described herein, utilizing information based on the existing secondary radar system, can be expected to obtain the bearing of a proximate aircraft with greater accuracy than that of the minimum TCAS-II. It may also surpass the directional accuracy of the enhanced TCAS-II. While achieving excellent directional accuracy, the invention, described herein, will avoid the large cost--expected to be several thousand dollars--of the TCAS-II's directional antenna.
Because each TCAS-II and each enhanced "active" TCAS-I acts as an airborne secondary radar, issuing interrogations and eliciting responses from aircraft in its vicinity, its use is expected to increase the rate of false targets received by the ground-based secondary radars which supply surveillance data to air traffic controllers. As terminal area radars are generally located at airports, and as the airspace surrounding airports is generally the most crowded, this interference is apt to be worst for radars at busy terminals, where the potential for mid-air collision is greatest, and where it is most critical that air traffic controllers receive high quality radar data. The FAA has attempted to limit this unwanted side effect of active TCAS operation by limiting TCAS transmissions in congested airspace, and predicts that the decline in performance of ground-based secondary radars will not be unacceptable. Because the avionics used in the method described herein is entirely passive, use of the invention would not in any way increase the false target rate experienced by secondary radars. Thus the invented method could be used instead of active techniques, or as a supplement to active techniques, to avoid the degradation of the air traffic control system that could be caused by simultaneous operation of many active TCAS units in densely populated airspace.
In summary, the passive systems heretofore developed are incapable of providing sufficient accuracy to operate without a prohibitively annoying "false alarm" rate, and are unlikely to meet with acceptance by the aviation community. The active systems heretofore developed must limit their operation in precisely those areas where collision avoidance service is most needed, the crowded airspace in the vicinity of airports; and those providing angular guidance will be available only at a cost so high as to discourage their purchase by the overwhelming majority of aircraft owners. The invention described herein would provide needed accuracy without increasing frequency congestion in terminal areas. Its compatibility with the FAA-developed TCAS allows it to be applied whenever or wherever it may be desirable to do so; and in principle, all hardware, software, installation and maintenance costs of its application could be supported by the private sector. By taking advantage of the technology of the existing secondary radar system through a novel utilization of proven hardware components, the invention provides collision avoidance service at a cost low enough to encourage its public acceptance.